1. Field of the Invention
The present invention relates to a magneto-optic recording and regenerating process for recording and regenerating information by employing the magneto-optic effect, a device using the process, and a magneto-optic disk.
2. Description of the Prior Art
With high degree of information, information file devices of higher capacity and density has been required and the magneto-optic disk unit has attracted attention as an erasable high-density recording device.
FIG. 2 is a view schematically showing the magneto-optic recording system in a conventional magnetio-optic disk unit, in which laser light beams 14 condensed by an objective lens 27 are applied to a perpendicular magnetic film 11 which has been magnetized normal to the plane of the magnetic film with its surface facing toward one direction, namely upward in FIG. 2, so that the temperature of the irradiated areas will be elevated in the vicinity of the Curie point at least or higher than the compensating temperature to reduce the coercive force of the film 11. Furthermore, the direction of magnetization is reversed by impression of an external magnetic field facing downward in FIG. 2, namely, in the direction opposite to the magnetizing direction of the perpendicular magnetic film 11 in respect of a magnetic field generating source 12 consisting of an electric magnet and the like, thereby storing information in the perpendicular magnetized film 11.
When erasing the recorded information in the perpendicular magnetic film 11, the external magnetic field in the same direction as the initial magnetizing direction of the perpendicular magnetic film 11 (upwardly in FIG. 2) as well as the condensed laser light beams irradiate the film, which will be then magnetized in the upward direction in accordance with the similar principle to that as in the recording, resulting in the information being erased.
The polar Kerr effect is used in regenerating the information recorded in the perpendicular magnetic film 11. A linear polarized light irradiated the perpendicular magnetic film 11 will turn into a reflected light rotating clockwise or anticlockwise in response to the magnetizing direction, and this rotation will be converted to the change of light volume by such as an analyzer to obtain a regenerated signal.
FIG. 1 is a schematic view showing one embodiment of a substantial part of a conventional magneto-optic disk unit; 41 identifies a magneto-optic disc using a perpendicular magnetic film. The magneto-optic disk 41 has information recording tracks formed concentrically or helically and turns at a high speed around its shaft, and a magnetic field generating source 12 such as electromagnet acts to apply a magnetic field to the magneto-optic disk 41 in a vertical direction.
A laser light beam produced by a semiconductor laser 23 runs through a condenser lens 24, a shaping prism 25 and a beam splitter 26 to be focused into and irradiate the perpendicular magnetic film of the megneto-optic disk 41 by means of the objective lens 27. If the temperature of the magnetic film is elevated by the laser light in the vicinity of the Curie point at least or up to over the compensating temperature, the perpendicular magnetic film will be magnetized in response to a direction in which the magnetic field generating source 12 impresses a magnetic field, whereby the information will be either recorded or erased.
When regenerating information, if a linear polarized light from the semiconductor laser 23 continues to travel through the condenser lens 24, shaping prism 25, beam splitter 26 and objective lens 27 until it irradiates the desired point of area on the perpendicular magnetic film of the magneto-optic disk 41, the polarization plane of a reflected light will revolve depending on the magnetizing direction. This particular polarized light is passed through the objective lens 27, beam splitter 26, another beam splitter 28, a 1/2 wavelength plate 31 and the condenser 32 and then separated by a polarizing beam splitter 34 into a component S of polarization and a component P of polarization. The intensity of light of the respective separated components are converted by means of photodiodes 33 and 35 into an electric signal. A differential ammplifier 36 produces a signal corresponding to the rotation of the plane of polarization in view of the difference between the two components, there-by the information will be regenerated, accordingly.
Part of the laser light beams irradiated at the time of recording, regenerating and erasing information is detected by a quadrantal photodiode 30 through the beam splitter 28 and a cylindrical lens 29 and the focusing and tracking of the light beam on the magneto-optic disk 41 is performed.
With such a conventional magneto-optic disk unit, in order to obtain a good recording sensitivity, the perpendicular magnetic film 11 needs to have a thickness of less than 1000 .ANG. in general so that a great demagnetizing field will be produced due to its own magnetization resulting from such a thinner quality, thus reducing recorded magnetization. In this connection, the perpendicular magnetic film 11 must be a substance of high coercive force in order to keep the recorded magnetization. What is known as a substance having such a nature at present is only an amorphous film made of rare earths-transition metals which has a great disadvantage in that it has a small Kerr effect and is subjected to oxidation and corrosion. This limits the selection of the magnetic film materials.
Additionally, the conventional magneto-optic recording system and the device using the system require the impression of a magnetic field perpendicular to the magnetic film, and the applied magnetic field diverges, whereby the distribution of the magnetic filed is not so limited that it is difficult to construct a compact source of generating magnetic field in which the high frequency modulation is possible.
Furthermore, a method for regenerating information in accordance with the conventional magneto-optic disk unit employs the Kerr effect, which allows the plane of polarization of a reflected light of laser beam irradiated the magneto-optic disk slightly rotate in response to the direction of magnetization of the magneto-optic disk. Since, however, a slight amount of rotation is converted by an analyzer such as a polarizing beam splitter light into the variation of the intensity of light to obtain a regenerating signal, which is too small to obtain a sufficient signal-to-noise ratio for regenerating information.
Converting the amount of rotation of the plane of polarization of reflected light into the variation of the intensity of light in accordance with the Kerr effect needs the optical polarization components such as a polarizing beam splitter, 1/2 wavelength plate and the like, as a result the optical system becomes complicated and it is difficult to reduce the size and weight of the device and therefore to give quicker access.
If the magnetized film is a perpendicular magnetic film, since, magnetization normal to the plane of the thin magnetic film is necessary, a great demagnetizing field will result from the magnetization of the film itself, thereby reducing the recorded magnetization. For example, if the saturated magnetization is 4.pi.Ms, it is necessary to provide a magnetic film having a greater coercive force than the saturated magnetization (4.pi.Ms) in order to overcome the demagnetizing field which will have an intensity of substantially 4.pi.Ms.
FIG. 3 is a graph showing the coercive force-temperature characteristic of a magnetic film in an approximate straight line, wherein the vertical axis shows the coercive force and the horizontal axis shows the temperature of the film. In the drawing, b, which represents the properties of the perpendicular magnetic film wherein a high corecive force Hc.sub.2 is required at the room temperature T.sub.R. In case of b, since the coercive force is at a high level in the room temperature T.sub.R, a comparatively steep gradient is shown by which the coercive force goes down to the Curie temperature Tc, and the film temperature Tex.sub.2 in which the magnetization reversal is enabled by the applied magnetic field Hex is relatively in the vicinity of the Curie temperature Tc. In general, the higher the recording sensitivity of the magnetic film, the easier the recording of information, and the recording sensitivity gets higher as the film temperature Tex enabling the magnetization reversal due to the applied magnetic field is at a lower level. Hence, for the purpose of getting high recording sensitivity, it is impossible to utilize the magnetic film in inherent possession of a high Curie temperature for the magneto-optic recording medium.
With a conventional magnetic film as shown in FIG. 3, line b, the film temperature Tex.sub.2 where the magnetization reversal is made possible by the applied magnetic field is seen to be in the vicinity of the Curie temperature Tc, and therefore, when a determined area of the magnetic film is heated to more than Tex.sub.2 by lasing so as to record or erase the information, it is inevitable that the film temperature will become higher than the Curie temperature Tc due to the energization. Consequently, during the time when the film temperature which has once become higher than the Curie temperature Tc goes down to a little lower than the Curie temperature, i.e. a temperature at which the magnetized wall in the magnetized region where the magnetization reversal took place can be fixed, since any recording or erasing of information cannot be achieved by the magnetization reversal of the magnetic film, for example, in the magnetic field modulation recording, a deviation from the recording or erasing position is liable to occur.
FIGS. 4 and 5 are schematic views showing the contigurations of magnetic domains where magnetic walls are fixed. As illustrated in FIG. 5, the configuration of magnetic domain which is symmetrical with respect to the running direction of the laser beam or the rotating direction the disk is the one which facilitates the processing of a regenerating signal, but the part of the magnetic domain heated to the temperature higher than the Curie temperature as described above has its magnetization which fails to reverse until the temperature is lowered to less than the Curie temperature, so that the formed magnetic domain as shown in FIG. 4 lacks the part heated to the temperature higher than the Curie temperature in the magnetic field modulation recording, thus being unsymmetrical with respect to a direction in which the laser beam runs or the disk rotates. Accordingly, when regenerating the information, a regeneration output will become unsymmetric in response to the configuration of the magnetic domain, making it difficult cult to process the regenerating signal.